In the last decade the knowledge of human photobiology has increased tremendously in the sense that it is clear that light radiation administered to a human subject through the eye—in addition to vision—is of major importance in controlling a variety of biological rhythms. Consequently, light radiation has not only an influence not only on many physical body functions but also on mental performance and mood.
Findings show a sensitivity of melatonin suppression for light radiation administered through the eye. Melatonin is a hormone showing a daily cycle and is considered a marker of the phase of the biological rhythm. During daytime the melatonin level is relatively low. The melatonin level increases in the evening, and reaches a maximum at night before it decreases gradually again to the minimum level during daytime, i.e. in the period a person normally is awake. Melatonin is generally known as a sleeping hormone that influences the alertness of the human subject. Hence, when the melatonin cycle is controlled, the risk on making mistakes because of lack of alertness is decreased. Suppressing melatonin in the natural daily cycle is possible in the usually ‘dark’ hours of the biological rhythm. Normally in this period only artificial illumination is available.
In a 24-hour society many people have to work and drive at night and be alert to perform well and safe, and to sleep well at abnormal hours. Under these conditions many people run an enhanced risk of making mistakes, for example causing car accidents, and/or are likely to suffer from a distorted sleeping behavior.
Sleep inertia and alertness dips are undesired from a performance or safety perspective. In general, sleep inertia persists for 30 minutes after waking up. For pilots and military, this persistence of sleep inertia may delay or even endanger operations.
WO 02/20079 discloses a method of controlling alertness of a human subject and a light source for use in this method. The method comprises exposure of a human subject during an exposure period to suitable light radiation. Experiments have shown that there is a particularly high sensitivity to light in the region of 420-460 nm, i.e. in the blue part of the photo spectrum.